Control loop systems are well known. Typically in such systems a component of the system, the plant, is controlled by a controller. By providing the controller and the plant in a feedback configuration it is possible to reduce discrepancies between the output of the plant and the desired output of the system. As well as these discrepancies, in all control loops there is uncertainty about the exact nature of the plant. The control loop reduces the effect of this uncertainty, but designing the control loop to be robust in this way requires some compromises.
Using an adaptive controller is another way to handle uncertainty in the plant. Typically, adaptive controllers can handle a wider range of uncertainty compared to a standard control loop. When the desired output is static for most of the time, then the control loop is normally called a regulator. Adaptive control loops or regulators are generally provided either in a direct or indirect configuration. Both configurations require knowledge of the system identifiers of the plant. Whereas the traditional direct self-tuning regulators use estimates of these system identifiers as an input to the regulator, the indirect self-tuning regulators involve a two-step process of (i) system identification of the plant parameters; and (ii) automatic design of the control parameters based upon the estimated plant parameters.
Estimation of the plant dynamics, by either the direct or indirect method, requires operation of the control system in either closed-loop or open-loop configuration. Persistent excitation is an issue for closed-loop system identification. As the system is in regulation, the dynamics of the loop are not exercised persistently, thereby impeding identification of the loop dynamics. An open-loop approach, whilst not suffering from the problem of persistent excitation, has an inherent issue in that the loop regulation breaks down during system identification and there is therefore the possibility of transients during the system identification stage corrupting the system. Open-loop operation is unsuitable for a system which continuously adapts to environmental parameter changes. On the whole, closed-loop estimation is preferred, despite the issues of persistent excitation.
Accordingly, the traditional self-tuning regulators have a requirement to identify the parameters of the plant in a closed-loop control system in which there is a lack of persistent excitation. They typically operate on the principle of certainty equivalence in which the estimated plant parameters are considered to be the correct values for the purposes of designing the controller parameters. Therefore, there is a requirement for the plant parameters to be estimated accurately, and for the estimated parameters to converge to the correct values as quickly as possible. System identification algorithms which meet this requirement are computationally complex, and therefore not suitable for a low cost ASIC implementation.
There is therefore a need to provide a control loop system that may be implemented in a closed loop configuration and yet provides for an estimation of the desired control parameters with low computational complexity to enable operation of the control system.